Selection bias ~= natural selection.

If bigmouth buffalo procreate for more years because they live longer with stronger immune systems, then they will eventually make up the majority of the population.

So long as they remain capable of reproduction in those later years

Is that so? I thought it’s enough if they provide an advantage for their genes.

Maybe healthier older fish protect their offspring more effectively.

Yes, you’re 100% correct. You’ve described kin selection. So if grandparent fish can somehow help their grandchildren procreate more, then that can be selected for.

I’ve heard of territory defense helping with fish, but no idea about the fish mentioned specifically.

Also to the previous comment, I think there’s already a fair bit of evidence that older larger fish do continue to breed, (hence throwing back the bigger fish breeders).

But if they stop breeding, and there is no kin selection; then yes, natural selection wouldn’t play a role.

Or if they care for their descendants in their later years.

(Not saying fish do this, though I wouldn’t be surprised if giant elder fish clear out predators. A lot of people forget this bit but it’s suspected of being a big factor in insect and lobster longevity, which both capable of recognizing their offspring and not being territorial to their presence.)

Or something changed in the environment that made younger fish less healthy.

You may be into something. I’ve been on the fish keeping hobby for decades and right now the number 1 cause of mortality of captive fish is immune system overload.

what does immune system overload mean as you've observed through fish keeping

Do you really think the researchers aren't aware of survivorship bias?